1. Field of the Invention
The present invention relates to an image processing method and devices adapted to embed invisible sub-information (e.g., security information) into visible main image information (e.g., an image of a human face) to produce composite image information and, at a later time, restore the sub-information from the composite image information.
2. Description of the Related Art
With the recent spread of digital information and the Internet, techniques of digital watermarking, digital signatures and the like have come to be regarded as important in order to prevent forgery or alteration of images. In particular, digital watermarking techniques, in which invisible sub-information is embedded in visible main image information, have been proposed as measures against unauthorized copying, forgery or alteration of identification (ID) cards or photographs having copyright information embedded.
For example, JP-A 2001-268346 (KOKAI) (document 1) and JP-A 2005-184603 (KOKAI) (document 2) each disclose a digital watermarking technique of embedding sub-information by superimposing the sub-information upon main image information utilizing color difference information.
Such a digital watermarking technique, which involves embedding to-be-embedded information, such as sub-information (digital watermarks), into main image information in a invisible form, requires a sub-information restoration process at a later time.
Conventionally, there have been various techniques to restore sub-information. Among these is a frequency filtering technique utilizing frequency domain.
In general, the digital watermarking technique is supposed to handle digital data and hence involves analog-to-digital and digital-to-analog conversion in restoring sub-information (digital watermark information) from a digitally watermarked image restored by an image recorder (a color printer or the like). It is therefore difficult to avoid the occurrence of degradation of the image quality. For this reason, there is a demand for a technique to minimize the degradation of sub-information in restoration.
With the method disclosed in document 2 which involves embedding digital watermarks by superimposing to-be-embedded information on main image information, the restoration of digital watermarks tends to be affected by the color and the frequency distribution of the main image information.